**2.1. Antigen factors**

of very large economic losses to poultry industry [3, 4]. Among bacterial, viral, parasitic and fungal diseases, the outbreak of viral diseases can cause havoc to the poultry industry causing reduced meat and egg production. The important viral diseases of poultry include Newcastle disease (ND), avian influenza, infectious bursal disease (IBD), infectious bronchitis (IB), etc. The high prevalence of diseases creates major constraints in the development of poultry sector. Immunization is the use of a biological preparation in the form of vaccine for enhanced immunity and prophylactic measures against specific diseases [5]. The process of injecting the vaccine in the body is known as vaccination. Proper vaccination can prevent losses due to diseases in poultry flocks [6]. Mostly, the vaccines are carried out against viral diseases but vaccines against salmonella, mycoplasma and coryza infections are also available. The vaccines against parasitic infections like coccidiosis are also being tested in different countries.

Vaccination is one of the most important tools for preventing diseases and in reducing the economic losses of the poultry producers [2]. Vaccination comprises the use of attenuated, killed, or recombinant organisms for stimulation of the body's immune response that recognizes the injected organism as a foreign antigen, resulting in clearing the antigen and developing memory cells in the body. Vaccination is the cheapest, reliable, effective, economical,

Live vaccines comprise a virulent virus whose pathogenicity has been weakened through consecutive cultures in living cells but the virus maintains its immunogenic antigenicity for stimulating the body's immune response; this whole process is commonly known as attenuation [7]. Commonly used live vaccines against diseases of poultry are Newcastle disease,

Killed vaccines comprise viruses whose pathogenicity has been inactivated through the use of physical and chemical means, but the protein coat structure has been maintained, which acts immunogenic. The viruses are physically inactivated by the use of ultraviolet radiations and heat and through chemical means by the use of formalin [9]. Killed vaccines against Newcastle disease and Avian Influenza are being used and have an advantage of providing

Vaccine failure is the consequence of the inability of the chicken to develop adequate immunity after immunization or susceptibility of bird to field outbreak after administration of vaccine [3]. High rates of 53.5% of vaccination failures have been recorded in vaccinated poultry flocks. Rates of 25.6, 25.6 and 2.3% of vaccine failure in Newcastle disease, infectious bursal disease (Gumboro) and fowl pox, respectively, have been recorded [2]. The common breaches in transportation, handling, storage and administration of vaccines are responsible for high

The causes of vaccine failure can be categorized into two major factors: antigen factor and

affordable and suitable alternate for prevention of diseases in poultry flocks [5].

infectious bronchitis, infectious bursal disease, etc. [8].

80 Immunization - Vaccine Adjuvant Delivery System and Strategies

rates of vaccine failure in poultry flocks in developing countries [2].

long term immunity to flocks.

**2. Causes of vaccine failure**

host response.

The protective vaccine antigen is of prime importance in the production of effective vaccine. The vaccines available in the market may have the following shortcomings resulting in vaccine failure.
